1. Technical Field of the Invention
The present invention relates to integrated circuits and, more particularly, use of active integrated circuit stubs in a point-to-point system of integrated circuits.
2. Background Art
FIG. 1 illustrates a system 10 of integrated circuits including integrated circuits IC0, IC1, IC2 . . . ICn. The integrated circuits are connected through a bus 16. Integrated circuit IC0 may be a controller to control IC1, IC2 . . . ICn or can be the same as them or can be the same as other integrated circuits or can be different. System 10 may be a system that is referred to as multi-drop system in which one or more of integrated circuits IC1, IC2 . . . ICn are selectively joined to bus 16 through stubs 18-1, 18-2, . . . 18-n, respectively. Merely as an example, system 10 may be a memory system in which controller 14 is a memory controller (either joined with a processor on chip or in a different chip from the processor) and in which integrated circuits IC1, IC2 . . . ICn are, for example, dynamic random access memory (DRAM) chips that are on one or more printed circuit boards (PCBs). They could also be in a multi-processor system.
Address, data, and control bits on bus 14, may be on separate conductors of bus 14 or they may be, for example, time division multiplexed or packetized. For example, bus 14 may include some conductors used to carry only address bits, some conductors used only to carry only control bits, and some conductors used to carry only data bits. Or, some conductors may be used to transmit some combination of address, control, and/or data bits at different times or through a packetized or arrangement.
FIG. 2 illustrates a system 20 including integrated circuits IC0, IC1, IC2 . . . ICn. The integrated circuits are connected through busses 24-1, 24-2, . . . 24-n. IC0 may be a controller to control the other integrated circuits or can be the same as them. System 20 is of the type that is sometimes referred to as point-to-point bus system. An optional conductor 28 completes the loop. System 10, by contrast, is referred to as a radial bus system.
A disadvantage of radial bus systems is that they are slower do to the relatively large amount of capacitance associated with relatively long conductors and multiple receiving circuitry in IC0, IC1, IC2, . . . ICn. An associated disadvantage is that energy is dispersed through the stubs as well as through the bus. For example, in FIG. 1, at the junction of the bus 16 and stub 18-1, some electrical energy passes through stub 18-1 and other electrical continues through bus 18 past stub 18-1. This reduces the effective bus speed.
Another disadvantage of current point-to-point systems is that in many systems there is a set number of chips. Even if the number of chips can be added to by adding to the last chip, if a chip in the interior of the loop fails, the whole system is down. For example, in FIG. 2, if IC2 is defective, the entire system may be nonoperational. By contrast, multi-drop systems are typically relatively easy to add to and in many such systems, if one of the chips fails, the system as a whole can still operate. For example, in FIG. 1, if IC2 is defective, the rest of the system may be able to continue.
Bi-directional signaling refers to using the same conductors to transmit signals in both directions. For example, data may be transmitted either to or from an integrated circuit. If the same conductor is used in both directions, the signaling is bi-directional. The bi-directional signaling may be sequential or simultaneous. In the case of sequential bi-directional signaling, enable signals may be used to, for example, tri-state or turnoff unused drivers or receivers. In the case of simultaneous bi-directional signaling, the threshold voltage of the receiver may be changed depending on the state of the adjacent driver. For example, if the adjacent driver is transmitting a 0, the receiver threshold may be set to Vcc/4. If the remote driver is also 0, the threshold will not be met. If the remote driver is a 1, the Vcc/4 threshold will be met. If the adjacent driver is also transmitting a 1, the receiver threshold may be set to 3Vcc/4. If the remote driver transmits a 1, the threshold will be met and if it transmits a 0, the threshold will not be met.